A New Microencapsulation Technique Based on the Solvent

Chapter 17

A New Microencapsulation Technique Based on the Solvent Exchange Method

Downloaded by PENNSYLVANIA STATE UNIV on July 6, 2012 | http://pubs.acs.org Publication Date: February 16, 2006 | doi: 10.1021/bk-2006-0923.ch017

Yoon Yeo and Kinam Park* Departments of Pharmaceutics and Biomedical Engineering, Purdue University, West Lafayette, IN 47907 *Correspondingauthor: [email protected]

Protein microencapsulation is difficult because of the sensitivity of proteins to various stresses encountered during the encapsulation process and release period. In an attempt to overcome such difficulties, a new microencapsulation method has been developed based on an interfacial phenomenon between a polymer solution and an aqueous solution, which we call "solvent exchange." A dual microdispenser system consisting of two ink-jet nozzles was employed to test the concept. This article describes the concept and advantages of the solvent exchange method.

Introduction Controlled drug delivery can influence the performance of a drug by mani pulating its concentration, location, and duration (1). Since the emergence of early controlled release products in late 1960s, controlled drug delivery systems have evolved to such an extent that drug release can be modulated in various manners to comply with needs of the body. Potential advantages of controlled drug delivery systems include: (i) improving patient compliance by reducing the number of drug administrations; (ii) reducing side effects by maintaining the 242

© 2006 American Chemical Society

In Polymeric Drug Delivery I; Svenson, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.


243 blood level of the drug within the therapeutic range; (iii) improving drug efficacy by extending duration of drug concentration in an effective level; and (iv) providing opportunities for targeted drug delivery (2). Controlled release technology has been highly successful with low molecular weight drugs; however, controlled delivery of high molecular weight drugs, such as genes, peptides, and proteins has been difficult. Peptides and proteins constitute an important group of therapeutic compounds. The high specificity and potency are major advantages of protein drugs as compared to traditional low molecular weight drugs (2). Advances in biotechnology have made recombinant peptide and protein drugs available in large quantities. Moreover, recent completion of the human genome project is expected to bring discovery of new protein drugs with superb bioactivities. Currently, most protein drugs are administered via invasive parenteral routes on a regular basis because the oral delivery of a protein drug is not a viable option at present. In this regard, use of biodegradable polymeric microparticles that can release a drug at a controlled rate for a specified period has been considered as an attractive alternative to thefrequentparenteral administration method. A number of microencapsulation methods have been developed during the past few decades. Advances in drug delivery technologies led to successful launch of commercial products on the market, such as Lupron Depot® (Leuprolide acetate, TAP Pharmaceuticals Inc.), Zoladex® Depot (Goserelin acetate, AstraZeneca), Sandostatin LAR® Depot (Octreotide acetate, Norvatis), and Trelstar™ Depot (Triptorelin pamoate, Pfizer). However, such successes are mostly limited to low molecular weight drugs or oligopeptides. Despite more than 20 years of effort to develop protein-encapsulated microparticle systems, only one product has reached the market: Nutropin Depot® (Human growth hormone, Genentech Inc.), which was approved by the U.S. Food and Drug Administration in 1999. Major challenges in protein microencapsulation come from difficulties in preserving structural and functional integrity of the encapsulated protein throughout the lifetime of the product, which result in undesirable release profiles as well as protein instability problems (3,4). Numereous studies discovered that proteins are sensitive to mechanical and chemical stresses and can easily be destroyed during the microencapsulation process and the prolonged release period. The most widely recognized problem in the contemporary microencapsulation techniques is that they can generate stressful conditions during the fabrication process and/or the release period such as (i) extensive exposure of proteins to a large water/organic solvent (w/o) inter facial area during microencapsulation process (5,6), (ii) mechanical stresses such as emulsification or homogenization (7), and (iii) extended contact with hydrophobic polymers and their degradation products (8). Proteins exposed to such damaging environments during fabrication of microparticles and/or the release period tend to undergo various structural modifications, leading to loss of



244 their biological functions or making them unavailable for release. As a result, a successful control of the release profile using the microparticle systems has seldom been achieved. Our interest in developing a new microencapsulation system is based on the hypothesis that protein stability and the release profiles can be unproved by minimizing those damaging conditions. The objective of this research is to develop a simple and efficient microencapsulation method that is distinguished from contemporary techniques. This new approach minimizes the formation of w/o interface, employs a mild instrumental system that is highly compatible with stress-sensitive drugs, and generates reservoir-type microcapsules that reduce the contact between encapsulated drugs and the potentially damaging environments. Solvent Exchange Method The new microencapsulation technique, which we call the "solvent exchange method," is based on a hypothesis that interfacial mass transfer bet-ween two contacting liquids can be utilized for making reservoir-type microcapsules. Figure 1 describes one method of making microcapsules, using two separate droplets based on the solvent exchange method. Microcapsules can be made as a layer of polymer solution encapsulates an aqueous droplets and then leaves a polymer membrane on the aqueous surface. Due to the surface tension gradient, most organic solvents that dissolve water-insoluble polymers can spread on the aqueous surface. Solid membrane occurs when interfacial mass transfer results in decrease in the solvent quality. In order to ensure this interfacial phenomenon, mutual solubilities of the two liquids are necessary. In order to provide a condition that allows contact between the aqueous droplets and the layer of polymer solution, we have developed a dual microdispenser system that consists of two ink-jet nozzles. This article presents how the new microencapsulation technique has been developed and the dual microdispenser system was used in implementing the new concept of micro-encapsulation.

Figure 1. Microencapsulation based on the solvent exchange method.


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Materials and Methods Screening of Organic Solvents Organic solvents that allow easy formation of a polymer membrane on an aqueous surface were screened as described previously (9). Briefly, poly(lactic acid-co-glycolic acid) (PLGA) was used as the encapsulating polymer, and organic solvents having a Hildebrand solubility parameter of 16-24 MPa were tested for (i) capability of solubilizing the PLGA polymer, (ii) diameter and (iii) turbidity of the polymer membranes that PLGA solutions of the solvents left on a 0.5% agarose gel. Downloaded by PENNSYLVANIA STATE UNIV on July 6, 2012 | http://pubs.acs.org Publication Date: February 16, 2006 | doi: 10.1021/bk-2006-0923.ch017

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Microcapsule Preparation by Dual Microdispenser System The dual microdispenser system consisted of two ink-jet nozzles (Figure 2). Microcapsules were produced as described previously (9). Briefly, a 2% PLGAethyl acetate solution and an aqueous solution containing a model drug and/or an excipient of choice were fed through each nozzle at a controlled flow rate. For confocal microscopy, FITC-dextran and Nile Red were added to the aqueous solution and the PLGA-ethyl acetate solution, respectively. The liquid streams were perturbed by a frequency generator (Hewlett-Packard model 33120A) to produce a series of droplets of uniform size. The trajectories of the two jets were precisely controlled to ensure collisions between every pair of droplets. The collision behavior was observed using a video camera under stroboscopic illumination. The microcapsules thereby formed were collected in a water bath. For comparison, microcapsules were also produced using the double emulsionsolvent evaporation method described in the literature (10), with slight modification. Particle Size Control Using an Ink-Jet Nozzle In order to understand the effects of instrumental parameters on size of the droplets formed by an ink-jet nozzle, nozzle orifice diameter d, forcing frequency f, and volumetric liquid flow rate Q were varied while introducing distilled water into an ink-jet nozzle as described previously (11). The drop sizes were determined from stroboscopic images. Apparently, the drops were highly homogeneous in size; thus, representative ones were taken to determine the size generated under the specific condition. In determining sizes of micro capsules, light microscopic pictures of the collected microcapsules were used. Reported values were averages of 30-50 microcapsules.



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Figure 2. Schematic description of a dual microdispenser system.

Microscopic Observation ofMicrocapsules Nascent microcapsules were observed using a bright-field microscope. A droplet of the microcapsule suspension was placed on a glass cover slip and the microcapsules were observed with a Nikon Labophot 2 microscope. The inter nal structure of the microcapsules was imaged using an MRC-1024 Laser Scanning Confocal Imaging System (Bio-Rad) equipped with a krypton/argon laser and a Nikon Diaphot 300 inverted microscope.

Results and Discussion Solvent Selection The new encapsulation method depends on the formation of polymer membranes on aqueous microdroplets; thus, it is important to find solvents which can dissolve the polymer (PLGA in this study) but form solid membrane upon contact with aqueous media. Thus, sixty organic solvents having solu bility parameters similar to that of PLGA polymers (i.e., 16-24 MPa ) were tested for their capability of dissolving the PLGA polymer. Dried PLGA (125 mg) was added to glass vials containing 5 ml of the test organic solvent. The vials were agitated overnight at room temperature. Solvents were classified into four groups: good solvents that formed clear polymer solutions; intermediately good solvents that formed turbid polymer solutions upon heating; intermediately 1/2


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poor solvents that were marginally able to swell the polymer; and poor solvents in which the polymer remained intact. The result is summarized in Figure 3, which was drawn after the method of Teas (12). It should be noted that only half of the screened organic solvents were able to dissolve PLGA. The rest were only able to marginally swell the polymer or were non-solvents. It is also noticeable that the good solvents form a reasonably well-defined area on the Teas graph. This result indicates that solubility of a polymer in a given solvent relies on the nature of intermolecular interactions as well as the closeness of the solubility parameters. Application of the Teas graph in the solubility prediction was described in detail in reference (9).

f

h

0

20

40

60

80

f

100

d

Figure 3. Comparison of Hansen's solubility parameters for various solvents: contributions of dispersion forces (fj), polar interactions (f ), and hydrogen bonding (f\). (0) Good solvent; ( ) intermediately good solvent; (V) intermediately poor solvent; and (X) poor solvent (Copyright 2003 Elsevier.) p

Selected solvents for PLGA were further refined by their spreading capability and the quality of the formed membranes. Solutions of polymer in different solvents were placed on a layer of agarose gel. The polymer films thereby formed were evaluated with respect to their diameters and optical densities, which were used to estimate the degree of spreading of each solution and the quality of the polymer membrane, respectively. That a polymer solution formed a membrane of a relatively large diameter implied that the solution would spread easily over an aqueous droplet in the encapsulation process. The


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solutions forming membranes which displayed relatively high turbidities were discarded, since it meant formation of a rough and discontinuous precipitate that would not be able to control the drug release. Therefore, solvents making poly mer solutions which formed membranes of large diameters and low turbidities were selected as desirable solvents, as indicated in Figure 4. Among the candi date solvents, ethyl acetate was used in following studies.

Figure 4. Evaluation of organic solvents based on their spreading over an agarose gel surface and subsequent formation of a polymer membrane. Preferable solvents lie in the shaded portion of the plot. Gray point indicates ethyl aceate. (Copyright 2003 Elsevier.)

Formation ofMicrocapsules by the Solvent Exchange Method Figure 5 shows the formation of microcapsules in air from collision and merger of two microdroplets generated by two ink-jet nozzles. The merged droplets were subsequently collected in a water bath to leave reservoir-type microcapsules. The geometry of the microcapsules, consisting of a single aqueous core and a polymeric membrane surrounding the core, was demon strated by confocal microscopy and clearly contrasted with those generated by the double emulsion method, as shown in Figure 6. It is noteworthy that the reservoir-type microcapsules having a continuous polymer membrane formed only when the solvents in the candidate area (shown



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Figure 5. Stroboscopic image of the microcapsule formation via midair collision between two component liquids. Here, the left stream is 0.25% alginate solution, and the right stream is 4% PLGA solution. Nozzle orifice diameter d = 60 ym; volumetric flow rate Q = 0.6 ml/min; Forcingfrequencyf = 10.6 kHz. Scale bar -100 pm.

Figure 6. Confocal laser microscopic images of microparticles produced by (A) the solvent exchange method and (B) the double emulsion-solvent evaporation method. Left and right panels indicate aqueous phases labeled with FITCdextran and polymer phase labeled with Nile Red, respectively.



250 in Figure 4) were used. Benzyl alcohol and acetic acid, for example, which lie outside the candidate area, were not able to form the membrane around the aqueous core or formed discontinuous membrane, respectively. Therefore, this result indicates that the criteria used in the solvent selection were valid guides for selection of appropriate solvents. Another interesting finding is that the size of microcapsules was relatively homogeneous, detennined by the interplay among different instrumental variables (11). The size of microdroplets produced by an ink-jet nozzle is a function of three variables: diameter d of the orifice, linear velocity of the jetted solution V (or volumetric flow rate Q), and forcingfrequencyf. In theory, the diameter dj of the microdroplets can be calculated by equating the volume of a fraction of the liquid jet (7i(d/2) X) emerging from the ink-jet nozzle and that of resulting spheres ((l/6)7id ): 2

3

d

d d

{

)

2f

\-p

In our previous study it was shown that the experimental values precisely agreed with the calculation (11). That is, the droplet size was primarily determined by the nozzle orifice, decreased with increasingfrequencyat a fixed flow rate, and increased with increasing flow rate at all tested levels of frequency. As for the size of microcapsules, which formed as a result of merging two equal-sized microdroplets, it was expected that the microcapsule diameter would be 1.26 times of the single droplets, assuimng that there was no loss of material upon their collision. The sizes measured from the stroboscopic pictures satisfied these expectations for both single and merged droplets as shown in Figure 5. On the other hand, the majority of microcapsules collected in the water bath were close to single droplets in size, and the membrane existed only as a thin membrane. First, it is possible that the polymer layer shrank as the solvent that constituted 95-98% of the polymer phase was extracted into the aqueous phases by the solvent exchange. Alternativerly, considering the existence of occasional satellites, it is also possible that portions of the polymer layer separated during stirring of the bath, leading to reduction of the membrane thickness.

Conclusions The solvent exchange method has been developed to address the traditional difficulties in protein microencapsulation. Reservoir-type microcapsules were produced, using a dual microdispenser system based on midair collision bet ween component materials, followed by interfacial phase separation of the



251 polymeric membrane. From the mild nature of the encapsulation process and the unique geometry of the microcapsules, it is expected that this method will provide several advantages over contemporary methods, especially in encapsu lation of proteins or peptides. First, the process does not include potentially damaging conditions such as an emulsification step, which often exerts unfavorable influences on the stability of encapsulated drugs by exposing them to the w/o interface and excessive physical stress. Second, in the mononuclear microcapsules, undesirable interactions between protein and organic solvent or polymer matrix are limited to the interface at the surface of the core. However, whether these potential advantages will be reflected through enhanced release profiles and stability of the encapsulated proteins remains to be seen. Third, the organic solvent for polymers can be chosen with more flexibility than in conventional methods; hence, toxicity concerns over residual solvents, in particular methylene chloride, can be avoided. Fourth, the use of ink-jet nozzles allows for a precise control over the particle size.

Acknowledgments This study was supported in part by the National Institutes of Health through grant GM67044, Samyang Corporation, Purdue Research Foundation, and NSF Industry/University Center for Pharmaceutical Processing Research.

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A New Microencapsulation Technique Based on the Solvent

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